1. Field of the Invention
This invention relates to glass compositions, and more particularly to (1) fiber glass compositions, and (2) mathematical relationships of the ingredients of the glass compositions to identify thermal properties of the glass compositions e.g., forming and liquidus temperatures with an acceptable temperature difference to prevent devitrification of the molten glass during forming of glass fibers.
2. Discussion of the Technology
In the art of forming glass fibers it is recognized that the difference between the forming temperature and liquidus temperature is maintained at a value to prevent devitrification of the molten glass during fiber forming. For example, the most common glass composition for making continuous glass fiber strands for textiles and glass fiber reinforcements is known in the trade as “E glass”. The requirements as to what type of composition constitutes an E-glass composition are included in ASTM D578-00. An advantage of using E-glass having 6-10 percent by weight B2O3 is that its liquidus temperature is well below its forming temperature, i.e. typically greater than 56° C. (100° F.) and usually in the range of 83 to 111° C. (150 to 200° F.). As used herein, the terms “forming temperature”, “TFORM”, “log 3 forming temperature” and “log3 FT” mean the temperature of the glass at which the viscosity of the glass is log 3, or 1000 poise, and the terms “liquidus temperature”, “liquidus T” and “TLIQ” mean the temperature at which the solid phase (crystals) and liquid phase (melt) of the glass are in equilibrium. The difference between TFORM and TLIQ, referred to herein as “delta T” or “ΔT”, is a common measure of the crystallization potential of a given melt composition. The lower the ΔT, in other words the smaller the difference between the forming temperature and the liquidus temperature, the greater the crystallization potential. In the glass fiber forming industry, ΔT is typically maintained at a temperature of at least 50° C. (90° F.) in order to prevent devitrification of the molten glass during a glass fiber forming operation, in particular, in the bushing area.
It is appreciated by those skilled in the art of forming glass fibers, that it is desirable to operate the glass fiber forming operation at low temperatures because operating the glass fiber forming operation at high temperatures results in high-energy usage, along with associated high-energy costs. In addition, the high temperatures accelerate degradation of the refractories used in the glass melting furnace, as well as the bushings used to form the fibers. The bushings include precious metals that cannot be recovered from the glass as the bushings wear.
It is recognized that boron and fluorine in addition to contributing to the electrical properties of the E-glass, e.g. dissipation factor and dielectric constant, also lower the forming and liquidus temperatures of the glass while providing a ΔT of at least 50° C. (90° F.). The boron in the form of an oxide and fluorine in the form of a fluoride are included in the glass batch materials and act as fluxes during the glass melting operation. E-glass can include up to 10 wt % B2O3 and up to 1.0 wt % fluoride (see ASTM D 578-00 §4.2). Although the boron effectively lowers the forming and liquidus temperatures while providing an acceptable ΔT value and the fluoride or fluorine acts as a flux, there are drawbacks. More particularly, these materials volatilize during melting and move with the exhaust gases into the exhaust system. If the boron and fluorine in the exhaust gases are not removed they are released to the atmosphere. Since boron and fluorine are considered pollutants, these emissions are closely controlled by environmental regulations, which in turn require careful control of the furnace operations and/or the use of costly additional pollution control equipment. In response to these concerns, the boron and/or fluorine content in E-glasses were reduced. For example, U.S. Pat. Nos. 4,542,106 and 5,789,329 and WO 99/12858 disclose boron free or low boron e.g. 1.8 wt % glasses having a forming temperature in the range of about 1258 to 1263° C. As can be appreciated by those skilled in the art of glass making, reducing or removing the boron and/or fluorine content in the batch materials raises the forming temperature. More particularly, the silica content increases as the boron content decreases, resulting in an increase in the forming temperature. As mentioned above, increasing the forming temperature results in high-energy usage, high-energy costs, and acceleration of the degradation of furnace refractories and bushings used to form the fibers.
For additional information concerning glass compositions and methods for fiberizing the glass composition, see K. Loewenstein, The Manufacturing Technology of Continuous Glass Fibres, (3d Ed. 1993) at pages 30-44, 47-60, 115-122 and 126-135, and F. T. Wallenberger (editor), Advanced Inorganic Fibers: Processes, Structures, Properties, Applications, (1999) at pages 81-102 and 129-168, which documents are hereby incorporated by reference.
In general, glass compositions having particular properties, e.g. forming temperature and delta T are identified by weight percent and/or molar percent of the components or ingredients in the glass. As can be appreciated by those skilled in the art of glass making and/or glass fiber forming, it would be advantageous to identify the performance of the glass compositions not only by the amount of a particular ingredient in the glass but also by a relationship, e.g. a mathematical relationship of the ingredients in the glass. WO 01/32576A1 discloses fiber glass compositions and the mathematical relationship of several of the ingredients, e.g. the combined Wt % of CaO and MgO, and the ratio of Al2O3/CaO, in the glass. Using the combination of glass ingredients, e.g. the absolute weight percent of silica with the relationship of other ingredients in the glass, e.g. the ratio of silica to RO (combined Wt % of CaO and MgO) would provide information regarding the melt properties of the glass.